Because the electrical resistances within plastics are usually very high, there is a risk of electrostatic charging, and this can be disruptive in certain application sectors, or can even be dangerous. Prior approaches for reducing internal electrical resistance of polymer resins include the addition of metal powders and metal fibers, carbon fibers, graphite, or carbon black. Electrically conductive carbon black has been practiced as a method for imparting electrical conductivity to many thermoplastic resins, including polyoxymethylene (polyoxymethylene). Examples of electrically conductive polyoxymethylene resin compositions are disclosed in U.S. Pat. Nos. 4,391,741 and 4,555,357.
As is well-known, loading of fine powders of carbon black into thermoplastic resins significantly reduces the toughness and flexibility of the final molded product. Deficiencies in part performance usually are a result of poor elongation and low impact strength. Disadvantages in the use of highly structured carbon blacks with polyoxymethylene is their sensitivity toward processing conditions, in particular inconsistent volume Resistivity due to lot-to-lot differences in work history imparted to the compound. Melt processing of polyoxymethylene and conductive carbon black leads to severe increase in melt viscosity, leading to degraded melt flow, and making it more difficult to form molded articles, especially injection molded articles where close tolerance limits with respect to electrical conductivity are required. On the one hand, the dispersion of the carbon black has to be sufficiently good, and on the other hand excessive shear must not be allowed to break down the agglomerates. For this reason, the challenge in use of lubricant additives is for controlling the degree of shear within the melt while at the same time avoiding loss of physical properties.
U.S. Pat. No. 6,262,165 discloses conductive polyoxymethylene, including 1.0-30 parts by weight of conductive carbon powder having a DBP absorption of at least 200 ml/100 g., 1.0-30 parts by weight of a carbon fiber of a specified average fiber length and 0.001-5 parts by weight of a lubricant. The lubricants are not limited. Specific mention is made of akali metal salts or alkaline earth metal salts of aliphatic carboxylic acids, amide compounds such as ethylenebisstearylamide, and aliphatic alcohols such as stearyl alcohol and behenyl alcohol.
U.S. Pat. No. 4,831,073 issued to Polyplastics Co. Ltd. teaches conductive polyoxymethylene made by forming a pre-blend of polyoxymethylene and thermal stabilizer, e.g., alkali metal carbonate, or and alkaline earth metal carbonates, prior to incorporating loadings of conductive carbon, e.g., carbon black, carbon fibers and/or graphite.
U.S. Pat. No. 4,828,755 describes a mixture where use of polyethylene glycol and non-polar polyethylene wax achieve incorporation of carbon black into the resin matrix. However it is noted that abrasion resistance, mechanical properties, and heat resistance, were compromised.
U.S. Pat. No. 6,790,385 teaches a conductive polyoxymethylene mixture including one lubricant having predominantly external, i.e., surface-active effect, and a lubricant with predominantly internal lubricant action, i.e. viscosity-reducing effects predominantly within the melt.
Owing to the problems attendant with polyoxymethylene in the use of conductive carbon black, including loss of physical properties, especially elongation at break, it would be desirable to reduce the effective amount of carbon black yet retain electrical Resistivity of 106 ohm-cm or less, and reduce variability of resulting properties obtained after injection molding.